The present invention relates to freeze drier shelves for supporting articles such as substances or vials or trays containing the substances within freeze driers. More particularly, the present invention relates to such freeze drier shelves in which the shelf also functions in the freezing and sublimation phases of the freeze drying process to freeze and heat the articles through circulation of a diathermic fluid through the shelf.
Freeze drier shelves are located within a freeze drying chamber of a freeze drier for supporting articles such as biological substances or more commonly, vials containing the biological substances to be freeze dried. The shelves are disposed in a vertical stack that may be collapsible in order to stopper the vials.
The shelves also serve to transfer heat between a diathermic fluid such as alcohol, glycol, mineral oil, and etc. and the articles to be freeze dried. During the freeze drying process, moisture present within the articles is frozen. After freezing, the articles are subjected to subatmospheric pressures that are low enough to enable the moisture to sublime into a vapor. To this end, diathermic fluid circulating within the freeze drier shelves is first cooled by an external refrigeration circuit in order to cause heat to be transferred from the articles to the diathermic fluid and thereby cause the freezing of the moisture contained within the articles. During sublimation, the diathermic fluid is slightly heated by an external heater in order to provide energy for the sublimation.
Since the freeze drying process occurs in a low pressure environment, heat transfer between the articles and the diathermic fluid occurs principally by conduction. As may be appreciated, it is critical that the shelves be as flat as possible in order to maximize the contact between the shelves and the articles. This maximization of contact in turn maximizes the degree of the conductive heat transfer between the articles and the shelves and hence, the diathermic fluid.
In the prior art, freeze dryer shelves are formed by two opposed stainless steels plates framed at the edges by a solid steel frame in order to form a space between the plates. Solid ribs traverse the space between the plates in order to form flow channels for the diathermic fluid. In one type of design, the ribs are longitudinally welded to the plates and are configured to interlock in order to form the flow channels when the plates and ribs are assembled. In another type of design, the ribs are simply welded to one of the plates. Holes are then drilled into the opposite of the plates and such plate is plug welded to the ribs. The resultant raised weld beads are ground flush and polished.
The problem with both types of of prior art freeze dryer shelf construction is that the welds will tend to thermally stress the plates in the vicinity of the welding. In order to reduce concomitant straining and thus, local deformation of the plates near the welding, very thick plates are used in fabricating the shelves and solid ribs are used in forming the flow channels for the diathermic fluid. The end result of the solid rib and thick plate construction of prior art freeze drier shelves is that each shelf possesses a sizable thermal mass or inertia. The result of this thermal mass or inertia is that a large fraction of the energy requirement of the freeze drier during the cooling phase of the freeze drying process is wasted in cooling the shelves.
In addition to the foregoing, the energy required in effecting the cooling is also wasted through heat leakage occurring during the cooling of the diathermic fluid. In the refrigeration circuit used in cooling the diathermic fluid, an external heat exchanger is provided to transfer heat from the diathermic fluid to a recoverable refrigerant such as FREON. Inevitably, there are thermal losses in the heat exchanger and the piping involved in conducting the cooled diathermic fluid back into the freeze drying chamber. As may be appreciated, such heat leakage must be compensated for by increasing the amount of refrigeration provided by the refrigeration circuit and thus, the energy required to provide the refrigeration.
As will be discussed, the present invention provides a shelf design that provides the requisite shelf flatness while having less thermal mass than prior art freezer shelf designs. In addition, a shelf design is provided in the present invention that minimizes heat leakage during the cooling of the diathermic fluid.